Typescript generic function for dynamic key and value

Question

const arrayToObject = <T>(array: T[], key: keyof T): Record<string, T> => {
  return array.reduce(
    (accumulator: Record<string, T>, currentValue: T) =>
      Object.assign(accumulator, {
        [(currentValue[key] as string).toString()]: currentValue,
      }),
    {} as Record<string, T>
  );
};

is there anyway to avoid using "as string"? how can I have type check that the value of property "key" in type T is always a string?

type T {
    someOtherProperty: 20,
    active: false,
   [key]: string; (the value type of the property "key" must be string)
}

jcalz · Accepted Answer · 2023-02-13 03:31:21Z

You could make arrayToObject() generic in both the array element type T and the relevant key type K, and constrain T so that its property at key K is assignable to string:

const arrayToObject = <T extends Record<K, string>, K extends keyof T>(
  array: T[], key: K
): Record<string, T> => {
  return array.reduce<Record<string, T>>(
    (accumulator, currentValue) =>
      Object.assign(accumulator, {
        [currentValue[key]]: currentValue,
      }),
    {}
  );
};

That compiles without error, and you can test that it works:

interface Foo {
  x: number,
  y: string,
  z: boolean
}

const foos: Foo[] = [
  { x: 1, y: "a", z: true }, 
  { x: 2, y: "b", z: false }, 
  { x: 3, y: "c", z: true }
];

const obj = arrayToObject(foos, "y"); // okay

Object.entries(obj).forEach(
  ([k, v]) => console.log(k, v.x.toFixed(1), v.y.toUpperCase())
);
/* "a",  "1.0",  "A" 
   "b",  "2.0",  "B" 
   "c",  "3.0",  "C" */ 

arrayToObject(foos, "x") // error!
// ---------> ~~~~
// Types of property 'x' are incompatible.

Looks good. The compiler will accept a correct call but complain if you pass it the wrong key (although it will say it's the wrong array, as it uses the key to check the array element type and not vice versa... but the error is clear, foos' elements don't have a string-valued x property).

Playground link to code

Matthieu Riegler · Accepted Answer · 2023-02-12 20:31:59Z

0

This will do :

const arrayToObject = <T extends Record<any, string>>(array: T[], key: keyof T): Record<string, T> => {
  return array.reduce(
    (accumulator: Record<string, T>, currentValue: T) =>
      Object.assign(accumulator, {
        [currentValue[key].toString()]: currentValue,
      }),
    {}
  );
};

Playground

answered Feb 12, 2023 at 20:31

Matthieu Riegler

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1 Comment

Lucas Chan Over a year ago

Hi, I forgot to mention that there are other properties which are not string type, I only specifically want the property "key" to be string type

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